It is well known in the art that the accuracy of an ADC is ultimately limited by the fact that it cannot by itself respond to analog signals less than half a least significant bit (LSB) in amplitude and that its response thereafter is in a series of steps, each equal to one LSB.
A method of reducing the effect of these step errors on the accuracy of ADCs in a digital oscilloscope system using repetitive analog input signals is described in UK patent no. 2067373 which discloses means for combining a dither pattern generated by a dither generator with a repetitive analog input signal prior to digitizing the analog signal. The dither pattern comprises a sequence of voltage levels corresponding in magnitude to different multiples of the LSB produced by the ADC. Each voltage level of the sequence of voltage levels is generated sequentially, clocked in response to the repetition of the analog input signal, and then combined with different repetitions of the analog input signal.
ADCs are now being used in video systems for digital processing of the video signal. Video ADCs also suffer from problems due to the step errors, however, in video systems the effect of the step errors is more pronounced in that the step errors can be seen on the picture screen and so reduce the quality of the picture.
The existence of step errors of video ADCs can best be seen on a TV screen when carrying out a horizontal grey scale test for picture linearity. At each step error. the graduality of the grey value will be disturbed and since this error occurs for conventional ADCs at the same horizontal position on each scanning line of the picture, an observer will be able to identify vertical lines which are caused by the step error of the video ADC. The effect is further accentuated by the horizontal edge enhancement circuit of the video amplifier. The edge enhancement circuit increases the bandwidth so as to sharpen the transients at the edges of the picture: otherwise the sharp transients at the edge of the picture result in bandwidth loss and blurred edges. However, the edge enhancement circuit also amplifies the error of the ADC across the whole line.
The above referenced UK patent relates to the generation of a dither pattern in response to the repetition of an analog signal: a video signal is not a repetitive signal since each line of a picture is made up of different levels of black and white. This patent does not address nor provide a solution to the specific problems which arise when converting video analog signals to a sequence of digital values.